In a case where a signal converted into a digital code is returned to an original analog signal (audio signal, for example), and the restored analog signal is further changed to a digital code and recorded on a recording medium (for example, a digital audio tape - hereinafter called "DAT"), the analog signal is often masked by "noises" of an amplifier, etc. when its signal level is low. Due to this, an analog-to-digital converter cannot maintain its original level resolution (16 bits in a DAT), and invites deterioration in the digitalized recording signal.
Generally speaking, if any noise enters in an analog signal, expected level resolution is not sufficiently effective in analog-to-digital conversion of the analog signal. In most cases, if a digitalized signal is converted to an analog signal, the converted analog signal itself includes less noise to establish high signal-to-noise ratio. However, noises often enter in the signal before subsequent digital recording.
The solid line curve in FIG. 5(a) shows an analog signal S. When it is converted to a digital signal, its bit levels represent the distribution shown by the solid line in FIG. 5(c). If a noise N shown in in FIG. 5(b) enters in the signal before analog-to-digital conversion, the analog signal takes a waveform S.sub.2 shown by the dotted line in FIG. 5(a) which has a considerable difference from the solid line curve. The difference does not matter to human ears if the signal level is high. However, human ears hear it if the signal level is low.
The analog signal may be amplified by a low noise amplifier before analog-to-digital conversion so as to subsequently convert the amplified analog signal to a digital signal, eliminating low level bits of the digital signal. For example, when an input analog signal shown by the solid line of FIG. 4(a) is applied to an analog-to-digital converter, the signal level in a period T is low, and a difference in period T is sensed by human ears. Therefore, the signal is amplified as shown by the dotted line in FIG. 4(a) (doubled in the illustrated example). This increases the amplitude in period T, and hence reduces the influence of the noise. However, since this amplification extends to the entire length of the signal, the hatched region, for example, overlaps the non-linear region of the amplifier and invites clipping or distortion caused by saturation.